1. Field of the Invention
The present invention relates generally to sensor arrays for acoustic sensing systems, and more specifically, to sensor arrays in which differences in the returned optical power are reduced.
2. Description of the Related Art
Arrays of fiber optic interferometric sensors show promise in applications where size, electrical interference, and electromagnetic detection make electronic sensors impractical. Such interferometric sensors are capable of measuring a parameter (i.e., a measurand) with a very high dynamic range (e.g., 120 dB). Optical sensor arrays are formed by connecting a series of sensors using fiber optic lines. If each sensor in an array requires a dedicated fiber to carry the detection signal, the large number of fibers required quickly becomes unwieldy as the number of sensors increases.
Optical couplers are commonly used to couple optical signals from a distribution bus to the sensors, and from the sensors to a return bus. In an array in which amplifiers are not used to periodically boost the signal strength, delivering sufficient optical signal to each of the sensors can be problematic. In addition, there may be significant variations in the optical power levels returned to the detectors, thereby complicating the signal processing.
Although significant progress has been made in recent years with respect to the sophistication of sensor array configurations and signal processing techniques, there is still need for improved architectures based on standardized components such as 1xc3x972 couplers, in which the returned optical signals have respective powers within a relatively narrow range.
One embodiment of the invention is a method of multiplexing optical signals for a sensor array. The method includes the steps of providing a first plurality of sensors having respective inputs and outputs, connecting a first plurality of input couplers to the inputs of the sensors, connecting the input couplers to respective distribution fiber lines which carry respective optical signals having different carrier frequencies, connecting a first plurality of output couplers to the outputs of the sensors, connecting the output couplers to a first return fiber line, and selecting coupling ratios of the output couplers to reduce differences in returned optical signal power levels.
Another embodiment of the invention is a method of multiplexing optical signals for a sensor array. The method includes the steps of providing a plurality of sensors having respective inputs and outputs (in which the inputs and outputs are connected to respective input and output couplers), providing distribution fiber lines which carry respective optical signals having different carrier frequencies, connecting the input couplers to the distribution fiber lines so that each distribution fiber line is only coupled to sensors that are non-adjacent, connecting the output couplers to at least two return fiber lines so that each return line is only coupled to sensors that are non-adjacent, and selecting the coupling ratios of the input and output couplers to reduce differences in the returned optical signal power levels.
A further embodiment of the invention is a sensor array that includes distribution fiber lines, return fiber lines, and sensor groups. Each of the sensor groups in turn comprises sensors, input couplers, and output couplers. The input couplers and the output couplers are connected to respective sensors, in which each of the input couplers within any one of the sensor groups is connected to a different one of the distribution fiber lines. Further, each of the return fiber lines is connected to all output couplers within respective ones of the sensor groups. The coupling ratios of the input couplers and the output couplers in the signal array are chosen to reduce differences in the returned optical signal power levels. In a preferred embodiment, this apparatus is an mxc3x97n sensor array comprising m distribution fiber lines, n return fiber lines, and n sensor groups.
A further embodiment of the invention is a sensor array that includes distribution fiber lines and sensor groups. Each of the sensor groups comprises sensors, input couplers, output couplers, and return fiber lines. The input couplers and the output couplers are connected to respective ones of the sensors, wherein no more than two of the input couplers within any one of the sensor groups are connected to any one of the distribution fiber lines. Further, the distribution fiber lines are only coupled to sensors that are non-adjacent. The return fiber lines are connected to each of the output couplers, such that each of the return fiber lines is only coupled to sensors that are non-adjacent. Also, the coupling ratios of the input couplers and the output couplers in this signal array are chosen to reduce differences in the returned optical signal power levels. In a preferred embodiment, this apparatus is a sensor array containing m distribution fiber lines and n sensor groups, in which each of the n sensor groups each contains 2 m sensors.